It is generally known to provide a pneumatic classifier for separating light tobacco leaf particles from heavier stem particles. For example, one such device is disclosed in the Rowell et al U.S. Pat. No. 3,164,548 in which the intermixed light and heavy tobacco particles are projected into and across the separating chamber while forced air is directed upwardly through the particles and at a uniform velocity across the length of the separating chamber. The lighter leaf particles are blown upwardly while the heavier stem particles fall downwardly and onto a vibrating screen positioned at an inclined angle so that the heavier particles are discharged in a location adjacent the end of the separator chamber containing the inlet for the intermixed leaf and stem particles. It has been found that the classifier of this patent cannot be efficiently utilized in the earlier stages of the separation of threshed tobacco leaves because the leaf particles and stem particles tend to form tangled balls or matts, usually referred to as "bird nests," within the separator chamber.
These bird nests are formed because the intermixed light and heavy particles tend to attach themselves together and the resulting bird nests are light enough to be carried upwardly by the upwardly moving forced air in the separating chamber so that some heavy stem particles are carried upwardly and removed with the lighter leaf particles. In other instances, the bird nests are of sufficient weight that they drop to the lower portion of the separator chamber and carry some of the lighter leaf products out the stem discharge outlet and to the next processing stage. In any case, the formation of bird nests is undesirable because they interfere with the efficient separation of the particles in the separator chamber, and they also tend to block or clog the stem outlet passageway and they disburb smooth flow of process.
U.S. Pat. No. 3,608,716, issued to Lorne A. Rowell, also discloses a similar type of pneumatic classifier for tobacco including an endless foraminous conveyor belt extending across the lower portion of the separator chamber for removing the heavier stem particles. This patent also discloses a recirculating arrangement in which an air jet is positioned in the wall opposite the material inlet wall to again force the particles across the chamber and subject the particles to the upflowing air and thereby provide a second stage of separation. This second stage of separation is said to aid in preventing the formation of bird nests. However, it has been found that the moving conveyor belt in the separator chamber requires maintenance and periodic replacement at substantial cost. Also, the recirculating of the particles in the second stage of separation casuses degradation of the lighter leaf particles.